Sub-woofer



FIG. 1 is a front perspective view of the sub-woofer;

FIG. 2 is a front view thereof;

FIG. 3 is a back view thereof;

FIG. 4 is a side view thereof; and,

FIG. 5 is a top view thereof. 

The ornamental design for a sub-woofer, substantially as shown and described. 